diffraction

A wave undergoes diffraction when it passes through a slit. To produce a significant diffraction pattern, the wavelength of the wave must be comparable to the width of the slit. The wavelength of visible light wave is about 400 to 800 nm (1 nm = 0.000 000 001 m) whereas the head of the torch is about 1 cm to 10 cm. Obviously, the two sizes are not comparable; the wavelength is far less than that of the size of the torch.